The subject invention compensates for the degradation of picture quality due to focus and convergence changes as a result of temperature variation around TV projection lens and cathode ray tube (CRT) assemblies.
In a projection TV, there is a projection lens which focuses the image from a small CRT onto a large screen. A typical configuration is shown in FIG. 1 and comprises a small CRT 10, a projection lens 11, mirrors 12 and 13, a projection screen 14 all housed in a common cabinet 15. In practice, there are separate lens and CRT assemblies for each of the colors red, blue and green, and each of the assemblies project an image onto the common projection screen. The projection lens shown in FIG. 1 is illustrated in more detail in the cross-sectional view of FIG. 2. This lens is composed of a plano-concave lens 20, a condensing lens 21, a mirror 22, and a plano-convex objective lens 23. The planar surface of lens 20 is optically coupled to the face plate of the CRT. The optics of the lens 11 are mounted within a unitary molded plastic housing 24, and the lens elements 20, 21 and 23 are also made of plastic. In order to focus the lens and CRT assembly, the lens elements 20 and 21 are mounted in a rotatable barrel mount 25 within the housing 24. The mount 25, like the housing 24, is molded plastic. There is a molded internal spiral groove in housing 24 into which fits a projection 26 on the outer peripheral surface of mount 25. The projection 26 is drilled and tapped to receive a wingnut assembly 27 which extends through a slot in housing 24. FIG. 3 is a top view of the lens 11 showing the objective lens 23 in housing 24 and the wingnut assembly 27 extending out of the housing 24. The diagonal bias of the slot which follows the internal spiral groove is clearly visible in FIG. 3. Focussing the lens involves loosening the wingnut assembly 27 and sliding it in one direction or the other in the slot in housing 24 and then retightening the wingnut assembly when focus has been achieved. By sliding the wingnut assembly in the slot, the mount 25 is rotated and, as a consequence of which due to the projection 26 in the spiral groove, is also translated along the axis of lens elements 20 and 21. In an alternative embodiment, the lens element 20 is fixed in housing 24, and only lens element 21 moves with the mount 25 in the focussing operation.
The projection TV shown in FIG. 1 is known as a rear projection TV, but the principles of operation are the same as for a front reflecting projection TV. Moreover, the lens 11 shown in FIGS. 2 and 3 may be used in either type of projection TV system, but while a folded lens is shown, the principles of operation are equally applicable to non-folded lenses which are also commonly used in projection TV systems. A manufacturer of projection lenses of the type described in U.S. Precision Lens, Inc. of Cincinnati, Ohio.
As may be seen in FIG. 1, the distance between the objective lens element 23 of the lens and CRT assembly 10, 11 and the projection screen 14 is fixed. The focus of each of the red, blue and green lens and CRT assemblies is set at the factory by the procedure of loosening, sliding and retightening the wingnut assemblies as described above. A problem arises during warm-up of the TV set when the ambient temperature around the lens changes, thereby causing the focus to drift. We have determined by empirical tests that, for the projection lens described, a temperature rise of 10.degree. C. will change the focus position approximately 1.0". In the past, this change in focus has been accomodated in the factory focus adjustment by setting the lens to be focussed at a median ambient temperature between a cold condition and a warmed-up condition after the TV set has been on for a period of time. In doing so, the depth of field of the projection lens is relied on to provide a reasonably focussed picture throughout the operating temperature. This is not entirely satisfactory where critical focus is desired, and the problem is further aggrevated by the fact that there are generally three CRTs in the projection TV set and the ambient temperature within the cabinet is not everywhere the same.